1. Field of the Invention
The present invention relates in general to a short-pulse amplification technique which employs a laser oscillator that generates pulses with much larger chirps than in known fiber oscillators to reduce the complexity of the overall system design.
2. Description of the Background Art
The generation of femtosecond pulses with energies above 1 μJ from a practical fiber source requires one or more stages of amplification. To avoid excessive nonlinear phase accumulation in the amplifiers, large-mode-area fibers are employed along with chirped-pulse amplification (CPA). A typical fiber CPA system consists of an oscillator, a stretcher, one or more preamplifiers, a large-mode amplifier, and a pulse-picker to optimize the use of the available power and to lower the repetition rate to that appropriate for a given application. Much progress has been made in fiber CPA systems, but to date they have barely begun to supplant solid-state instruments in applications despite their major potential advantages. There is clear motivation to simplify the systems to better exploit the benefits of fiber, particularly greater integration and lower cost.
More particularly, known short-pulse fiber amplifiers employ a low energy, short-pulse, and high repetition-rate oscillator, the output of which must be substantially modified before the pulse can be amplified to a useful energy level. First, the stretcher is employed to stretch the output pulse form the oscillator to greater than 100 ps duration. This stretching is necessary to avoid deleterious nonlinear phase accumulation in the later amplification stages, and the pulse energy is kept low to avoid nonlinear phase shifts in the stretcher. The pulse picker is then used to cut the repetition rate to obtain more energy per pulse for a given average amplifier power, which is limited by available pump power. The modified pulse is then amplified and finally compressed back to close to the transform-limited duration (e.g. on the order of hundreds of femtoseconds). Several pre-amplification stages are generally needed to reach the desired overall gain, which can be as high as 107. The use of a low power oscillator, a pulse stretcher, a lossy and costly pulse picker and several stages of amplification represent costly and inefficient disadvantages to the otherwise desirable CPA based systems. A need clearly exists for a more cost efficient design of a CPA system.